This invention relates to a fluidized bed system in which bed material, such as coal, coal ash, limestone, cement clinker or sand, is burned, gasified or dryed by being fluidized.
In the conventional fluidized bed system, a plurality of fluidized bed cells of which operational conditions are different from each other are formed by providing partition plates. Usually, in the system, temperature is raised to 500.degree.-1000.degree. C., so that the partition plate has inevitably been complex in construction and high in cost because of the need to withstand the high temperature. Also, provision of a partition plate has made the fluidized bed system unable to meet various different operating conditions. In other words, the system does not have a wide use.
In a fluidized bed combustion system for burning, for example, coal or coal ash, it has hitherto been usual practice to keep the temperature of the fluidized bed in the range between 800.degree. and 1000.degree. C. to burn a fuel, such as coal, for reasons stated hereinafter.
(1) Burning the fuel at a high temperature of over 1000.degree. C. produces oxides of nitrogen (NOx) as the bonding of nitrogen to oxygen contained in the air used for combustion takes place. When the concentration of NOx emissions in the flue gas rises, the atmosphere is polluted and this would give rise to a problem with regard to environmental disruption.
(2) When a fuel of high sulfur content is burned, it is now usual practice to mix the bed material with limestone particles to remove sulfur oxides (SOx). To enable this desulfurization reaction effectively requires maintaining the temperature of the fluidized bed in the range between 850.degree. and 1000.degree. C.
When particles of coal or EP ash (ash collected by electric dust collectors) are burned in a fluidized bed, fuel particles of small diameter are scattered by gas, such as air, for achieving fluidization of the bed material, and fly to a hollow body above the fluidized bed, with the result that such fuel particles are conveyed to the flue by exhaust gases of combustion before they are fully combusted. Fuel particles of relatively large diameter are broken down to small particles by combustion in the fluidized bed. However, with the temperature of the fluidized bed being at a relatively low level of 800.degree. to 1000.degree. C., the fuel particles are conveyed to the flue by the exhaust gases of combustion through the hollow body before being burned to the core of each particle.
Thus exhaust gas from fluidized bed combustion systems contains ash of high non-combusted fuel component. Accordingly, the present practice is to separate the ash content of flue gas by a cyclone dust collector and to feed the separated ash into a re-combustion furnace of the fluidized bed system maintained at a high temperature of about 1100.degree. C. so as to achieve a complete combustion of the non-combusted fuel components contained in the ash to turn the same into ash of the type that can be utilized as fly ash. FIG. 2 shows one example of the system for re-combusting the non-combusted fuel components. The system comprises a furnace 1 of a main combustion chamber, a cyclone dust collector 2 for collecting solid masses of ash released from the main combustion chamber, an air heater 3 for supplying combustion gas of high temperature to a start-up wind box 4, when the main combustion chamber is started up, to heat bed material above the wind box 4, a wind box 5 for the main combustion chamber, heat transfer tubes 6 located in a fluidized bed 6 in the main combustion chamber, a furnace 7 of a re-combustion chamber for re-combusting solid masses of ash containing non-combusted fuel components released from the main combustion chamber and collected by the cyclone dust collector 2, a cyclone dust collector 8 for collecting ash released from the re-combustion chamber, an air heater 9 having the same function as the air heater 3 used when the main combustion chamber is started up and used when the re-combustion chamber is started up, a wind box 10 for the re-combustion chamber for supplying air for combustion to set in motion bed material forming a fluidized bed 11a and to achieve a complete combustion of the solid masses of ash containing non-combusted fuel components which are supplied to the fluidized bed 11a, an ash transporting tube 12 for transporting the solid masses of ash containing non-combusted fuel components collected by the cyclone dust collector 2 and feeding the same into the fluidized bed 11a through ash feeding nozzles 13 and an exhaust duct 14 for releasing exhaust gases to the atmosphere.
When coal is used as a fuel, for example, in the fluidized bed combustion system of the aforesaid construction, costs are high because it is necessary to use an additional fluidized bed combustion system for collecting and re-combusting solid masses of ash containing non-combusted fuel components.